The group warns there is a danger Lib-Dems will be portrayed as not trusting the British public to make a decision on Europe. They say the Conservative Party has stolen a march in painting themselves as the only party wanting a vote on leaving the EU.

A Lib-Dem source said: “The Tories are saying they are the only party offering a referendum and that we don’t want to give people a choice. The danger is the charge will stick.

“We don’t want to end in a position where David Cameron is being seen to offer a referendum and Nick Clegg is not giving the country a choice.”

David Cameron has said if the Tories win a majority at the 2015 election, he will hold a referendum in 2017 asking whether Britons want to leave the EU or stay in on renegotiated terms.

When questioned publicly on Europe, the Deputy Prime Minister has argued the Government should be focusing on different things. He points out ministers have already legislated for a referendum on any new treaty if it means further powers pass to the EU. But the senior Lib-Dems want the him to shift his party further from Brussels.

They disagree that he should continue arguing for Britain to be at the “heart of Europe” — the position he favours. While they want to remain inside the EU, they say membership should not be unconditional. Instead, they want to see their leader advocate a more realistic role for the UK taking account of its position outside the eurozone.

The source said: “Lib-Dems aren’t all as euro-friendly as people think. There are a significant number who thought it was catastrophic to commit the party to joining the euro. Looking at things now, Britain can’t be at ‘the heart of Europe’ because to be at the heart of Europe you need to be in the euro.”

Labour leader Ed Miliband is also under pressure to change his policy after a number of his backbenchers launched the Labour for a Referendum campaign. His plan is to only offer a vote in the event of a treaty change transferring sovereignty to the EU. They want a straight in-out referendum.